Antennas that are used in mobile communications networks, such as GSM, CDMA, TDMA, or UMTS are often designed as antenna arrays. An antenna array comprises a plurality of antenna elements that are distributed in a one-dimensional or two-dimensional manner. Each of the antenna elements transmits or receives basically the same signal. However, by introducing a different phase shift for each of the antenna elements, the radiation distribution of the antenna array, in particular its shape and its direction, can be modified up to a certain degree.
In a normal operating scenario, an antenna array is likely to have an unequal and predictable power distribution across its antenna elements. More power will typically be required for the central antenna elements and less for outer ones.
U.S. Pat. No. 5,504,493, entitled “Active Transmit Phased Array Antenna with Amplitude Taper”, issued to Hirshfield and assigned to Globalstar L. P. on Apr. 2, 1996 describes a phase array transmitting antenna system, including a plurality of radiating elements. One or more constant phase and amplitude amplifiers are affixed to the radiating element in the array, wherein the radiating element is capable of producing radiation having a certain phase and amplitude that is distinct from the phase and amplitude of radiation produced by most of the other radiating elements. The amplifiers need to track one another in both amplitude and phase transfer characteristics. U.S. Pat. No. 5,504,493 therefore suggests using substantially identical amplifiers. This means that especially the outer amplifiers have to be operated in a range of operation that is beneath that of the optimal range of operation for the amplifiers. Therefore, especially the outer amplifiers tend to show rather poor efficiency. In addition, amplifiers having higher power ratings are usually more expensive than those having lower power ratings. The entire disclosure of U.S. Pat. No. 5,504,493 is hereby incorporated into the description by reference.
U.S. Pat. No. 4,825,172, entitled “Equal Power Amplifier System for Active Phase Array Antenna and Method of Arranging Same”, issued to Thompson and assigned to Hughes Aircraft Company discloses the use of a plurality of equal-power RF power amplifiers attached to a plurality of antenna elements. Each RF power amplifier is utilised at a power level close to, or at, peak efficiency in such a way as to provide a range of transmitted power levels from the antenna elements. Each RF power amplifier is composed from a combined pair of identical amplifiers. Constructive/destructive interference is used in the combiner to provide the desired signal power level. The teachings disclosed in U.S. Pat. No. 4,825,172 find application in satellite communication which requires relatively high power levels. The antenna array disclosed in U.S. Pat. No. 4,825,172 is a linear, one-dimensional array. The entire disclosure of U.S. Pat. No. 4,825,172 is hereby incorporated into the description by reference.